1. Field of the Invention
This invention relates to a fuel supply control system for internal combustion engines, which controls the air-fuel ratio of a mixture supplied to the engine, and more particularly to a fuel supply control system of this kind, which controls the air-fuel ratio immediately after the start of the engine.
2. Prior Art
Conventionally, there has been proposed a method e.g. by Japanese Patent Publication (Kokoku) No. 6-63468, in which the air-fuel ratio of a mixture supplied to the engine is controlled to a leaner value than a stoichiometric air-fuel ratio (hereinafter referred to as "lean-burn control") until an exhaust gas-purifying device becomes active, or when the temperature of the engine is lower than a predetermined value, after the start of the engine, whereas after the exhaust gas-purifying device has been activated, or when the temperature of the engine exceeds the predetermined value, the air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio. This method makes it possible to reduce emission of noxious components, particularly hydrocarbons (HC).
According to the above conventional method, however, when lean-burn control is carried out immediately after the engine is started in a cold state, if gasoline with low volatility is used as the fuel, the combustion state of the engine can become worse or unstable, causing degraded driveability of the engine. Therefore, it is required to set a lean-burn correction coefficient to a value considerably richer than a lean limit beyond which the air-fuel ratio is not permitted to be leaned, so as to avoid degradation of the driveability of the engine. This makes it impossible to improve exhaust emission characteristics of the engine, such as reduced emission of HC, to a satisfactory degree as intended by the lean burn control.
Further, when the engine is not equipped with means for feedback-controlling the air-fuel ratio to a leaner value than the stoichiometric air-fuel ratio, or when the air-fuel ratio sensor is in an inactive state and therefore open loop control of the air-fuel ratio has to be carried out, it is also required to set the lean-burn correction coefficient to such values as can accommodate differences in the combustion state between engines due to manufacturing tolerances of component parts of the engine between production lots and/or expected aging of the component parts, which causes the same problem as stated above.
Therefore, the conventional method still remains to be improved in the manner of lean-burn control to achieve satisfactory exhaust emission characteristics of the engine, such as reduced emission of HC.